The early wind energy developments were planned, permitted, constructed, and operated with little consideration for the potential impacts to birds (Anderson et al. 1999). Observations of dead raptors at the Altamont Pass Wind Resource Area (WRA) (Anderson and Estep 1988, Estep 1989, Orloff and Flannery 1992) triggered concerns on the parts of regulatory agencies, environmental/conservation groups, wildlife resource agencies, and wind and electric utility industries about possible impacts to birds from wind energy development.
Bird fatality rates observed at most wind projects are not currently considered significant to individual bird species populations. Although many bird species have observed fatalities, raptors have received the most attention (Anderson and Estep 1988; Anderson et al. 1996a, 1996b, 1997, 1999, 2000; Estep 1989; Howell and Noone 1992; Howell 1995; Hunt 1994; Johnson et al. 2000a, 2000b; Luke and Watts 1994; Martí 1994; Orloff and Flannery 1992, 1996; and Thelander and Rugge 2000). The emphasis on raptors probably emerged for several reasons:
- Raptors appear to be more at risk for collision than other bird groups
- Raptors are symbolic and have emotional value to many Americans
- Raptors are protected under federal and state authorities, including the Federal Migratory Bird Treaty Act, and some are protected by the Bald Eagle Protection Act and Endangered Species Act; thus, companies risk violating the law.
Other WRA studies have documented deaths, primarily of songbirds (Anderson et al. 2000; Erickson et al. 2000, 2001; Higgins et al. 1995; Johnson et al. 1998, 1999, 2000a, 2000b; Orloff and Flannery 1992; Osborn et al. 1996; Pearson 1992; Thelander and Rugge 2000; Winkelman 1994) and waterbirds (Anderson et al. 2000; Erickson et al. 2001; Pearson 1992; Johnson et al. 2000a, 2000b, 2002; Winkelman 1985, 1989, 1990, 1992a, 1992b; Winkelman 1994). Bats also have been killed at wind energy facilities (Anderson et al. 2000; Erickson et al. 2000; Higgins et al. 1995, Johnson et al. 1997, 1998, 1999, 2000a, 2000b, 2003). Generally, bat fatalities have included migratory species, and until recently have not been subject to the degree of concern associated with avian fatalities.
Avian fatalities from human caused sources have been estimated near 1 billion a year in North America alone. Power lines, buildings and windows, communication towers, vehicles and pesticides are estimated to comprise more than 82% of the total mortality. At the current level of development, wind turbines are estimated to comprise less than 0.01% of the total annual avian mortality from human-caused sources (Erickson et al. 2004). Although this proportion is extremely small, potential impacts to species or groups of concern, and proper siting of individual wind projects and turbines within wind projects, still need consideration.
A potential cumulative impact from all sources is still a continued concern, since many bird populations are in decline throughout the United States (USFWS 2002).
The levels of concern about wind turbine impacts on birds will likely remain high until we have a better understanding of the factors related to bird fatality. Studies such as this will hopefully provide valuable information regarding avian use and fatality and help reduce the level of uncertainty with wind energy development.
The primary objective of this study was to estimate and compare bird utilization, fatality rates and collision risk indices among factors such as bird taxonomic groups, turbine types and turbine locations within the operating wind plant in the Tehachapi Pass WRA, in southcentral California between October 1996 and May 1998.